The present invention relates to a radio frequency filter using a surface acoustic wave (hereinafter referred to as xe2x80x9cSAWxe2x80x9d) resonator filter or a film bulk acoustic resonator (hereinafter referred to as xe2x80x9cFBARxe2x80x9d) filter suitable for a mobile radio terminal, a radio frequency circuit having such a radio frequency filter, an antenna duplexer using such a radio frequency circuit, and a radio terminal using such an antenna duplexer.
Conventionally, mobile radio terminals have used a heterodyne system. According to the heterodyne system, a received radio frequency (RF) signal is once converted into a comparatively high intermediate frequency, and then demodulated through an amplifier, a filter, and so on. In recent years, in dual band terminals, code division multiple access (CDMA) terminals, or the like, a direct conversion demodulation system or a low intermediate frequency demodulation system has been investigated for the purpose of simplification of RF circuitry.
Generally, in the direct conversion demodulation system or in the low intermediate frequency demodulation system, a differential low noise amplifier (hereinafter referred to as xe2x80x9cLNAxe2x80x9d) or a differential mixer (hereinafter referred to as xe2x80x9cMix.xe2x80x9d) is used. In the background art, however, the connection between the RF filter or the antenna duplexer and the LNA or the Mix, has not been taken into consideration sufficiently. For example, the reception-system output of the antenna duplexer is single-ended because the output is extracted directly from a built-in filter. Accordingly, mismatching may occur between the antenna duplexer and the differential LNA or the differential Mix.
To eliminate such mismatching, it is considered to be the most general procedure to use an unbalanced-to-balanced converter constituted by distributed parameter lines as shown in FIG. 1.
In FIG. 1, the reference numeral 41 represents an unbalanced input terminal; 42, a balanced output terminal; and 43, a quarter-wavelength coupler constituted by a quarter-wavelength microstrip line.
However, since this converter has quarter-wavelength lines as its basic configuration, the volume of the converter is large so as to cause an obstacle to miniaturization of the RF circuit as a whole.
Unbalanced-to balanced convert is described in the document xe2x80x9cProduct Hotline Magazine Vol. 34, January 2000, pp.17-22xe2x80x9d.
It is an object of the present invention to solve the problem that might arise from the connection between a radio frequency filter or an antenna duplexer and a differential LNA or a differential Mix. introduced in a direct conversion demodulation system or a low intermediate frequency demodulation system that will be adopted widely in the near future, that is, the problem that the volume is so large that the RF circuit as a whole becomes large in size.
It is another object of the present invention to provide a system different from the unbalanced-to-balanced converter in order to satisfy the specification in GSM Regulation 05.05 that is a regulation of GSM.
Generally, in an antenna duplexer for a mobile radio terminal, dielectric resonators connected in cascade have been used as a reception-system filter or a transmission-system filter. In the present invention, a ladder-type SAW resonator filter in which a plurality of SAW resonators are connected in a ladder circuit configuration in an SAW chip or a ladder-type FBAR filter in which a plurality of FBARs are connected in a ladder circuit configuration in a chip is used particularly in a reception-system filter so as to microminiaturize a radio frequency filter.
Next, a first reception-system output terminal of the antenna duplexer is extracted from an output terminal of the ladder-type SAW resonator filter or the ladder-type FBAR filter (collectively referred to as xe2x80x9cladder-type resonator filterxe2x80x9d) through a serial inductance and a parallel capacitance with respect to the ground. A second reception-system output terminal of the antenna duplexer is extracted from the same output terminal of the ladder-type resonator filter through a serial capacitance and a parallel inductance with respect to the ground. In addition, the values of the serial inductance, the parallel inductance, the serial capacitance and the parallel capacitance are determined to be specified values on the basis of the input impedance of a differential LNA or a differential Mix. in a subsequent stage and the output impedance of the ladder-type resonator filter.
As a result, the specification is designed to satisfy the above-mentioned GSM regulation. Thus, in the pass band frequency of the ladder-type resonator filter, the amplitude deviation between output signals from the first and second terminals can be fit to a value not larger than xc2x11 dB, and the phase deviation between the output signals from the first and second terminals can be fit to a value not larger than xc2x110xc2x0 with respect to 180xc2x0. Accordingly, the antenna duplexer is allowed to connect in series with the differential LNA or the differential Mix. in the subsequent stage. Further, this configuration is also established on a mere ladder-type resonator filter alone in the same manner.